The present invention relates to exhaust systems, in particular to an exhaust system wherein scavenging occurs, especially in combination with feedback and silencing.
In internal combustion engines it is quite common for two or more of the exhaust valves of various cylinders to be open at the same time. Normally, the exhaust gas pressure in the cylinders with exhaust valves open at the same time will not be the same. For example, for one cylinder the exhaust port may have just opened and the exhaust gases within the cylinder are at a relatively high pressure, whereas simultaneously the exhaust valve for another cylinder may have been open for some time such that most of the gases within the cylinder have escaped and the valve has started to close. If the gases escaping from these various cylinders are all vented into the same exhaust manifold, it is quite possible, due to poor engineering, that the exhaust gases from the cylinder where the valve has just opened may at least partially repressurize the cylinder where the valve is just about to close with previously combusted exhaust gases thereby substantially decreasing the efficiency of the engine.
Over the years various exhaust configurations have been designed in attempts to alleviate the above mentioned problem and thereby increase the efficiency of the engine. For instance, exhaust gases from cylinders which would have exhaust valves open at the same time were vented into separate exhaust manifolds. However, higher efficiency has been obtained when, through proper design, the exhaust gases escaping at high pressure from one cylinder are utilized to draw exhaust gases from a cylinder at relatively low pressure before the exhaust valve of the latter closed. This operation and related operations are generally referred to as "scavenging".
In theory, the performance (as used herein--the relative horsepower at any given rate of revolution of the engine) can be improved by decreasing the amount of combusted exhaust gases remaining in a cylinder prior to intake of fresh fuel. Also, in theory, performance can be improved if the fresh fuel (often in a relatively rarified state when entering the cylinder) can be compressed to put more fuel air mixture into the cylinder, as is generally the reason for adding a supercharger to an engine. It is desirable that the fuel-air mixture which is combusted in each succeeding fuel-air burn in a cylinder contain a minimal amount of the burnt fuel-air mixture of the preceeding combustion and as much of the fresh fuel as possible (as long as the fresh fuel remains gaseous). When it is not possible to have only a given fuel-air mixture in the cylinder, it is preferred that fresh ambient air be included therewith, rather than exhausted gases from a previous combustion. Therefore, one of the functions of the present invention is to reduce the amount of residual combusted gases in a cylinder for the succeeding burn as much as possible, while increasing the quality of fresh fuel in the cylinder (as measured by weight rather than volume).
Many types of engines, especially high performance racing and aviation engines tend to have a particular power output associated therewith that tends to peak at a particular engine speed (RPM) and falls off substantially on either side of such a peak (or sometimes peaks). This would not present a problem if these engines only operated at one speed; however, almost all race or aviation vehicles must operate over a wide range of speeds (or altitudes) and, hence, it is preferred that the engine be able to operate efficiently over a wide range of power outputs and vehicle speeds without constantly changing gears to maintain a constant engine output. Therefore, it is desirable to provide a more uniform power output at different engine speeds. This is especially true of engines that have a peak power band at a relatively high speed. Such engines must be kept at this high speed at all times in order to make best use of the peak power by switching gears which becomes very cumbersome to the driver and the high speed tends to wear heavily on the engine causing the engine to wear out over a relatively short period of time.
The peak power band of an engine normally appears to be related to characteristics of the engine and the exhaust system thereof. Normally, the increase of cylinders exhausting into an exhaust header has the effect (if the header is properly designed) of decreasing the engine speed (RPM) at which peak power occurs and/or tends to make the power output more equal over a wider range of engine speeds. Theories have been suggested as to why this occurs; however, the theory is less important to the present invention than the fact that it does occur.
Applicant has theorized that a feedback mechanism utilizing a slip stream of the sonic or mass wavefront of the exhausting gas (emissions) from a first exhaust pipe may be redirected to or up a second pipe toward the engine. In theory, under proper sizing (length of pipe, pipe cross-section, location of intersections, etc.) for a particular engine, the emission wavefront passes along or up the second pipe that is associated with a cylinder that has been over scavanged (that is, fresh fuel has been drawn through an associated exhaust port into the second exhaust pipe) and compresses the fresh fuel back into the cylinder just prior to the exhaust port valve closing so as to effectively supercharge the fresh gas into the cylinder. Where the wavefront is sonic, there is, in effect, a sonic supercharging of the fuel.
It is also desirable to be able to artificially increase the number of discharges through a particular discharge pipe to emulate a system wherein additional exhaust pipes are joined together.
It is also important in exhaust systems of this type to limit the amount of noise produced by the escape of gases from the end of the exhaust system into the ambient atmosphere. Various types of silencing devices have been developed over the years which use different techniques to deaden the noise escaping from the exhaust system utilizing same.